The present invention relates to drilling subterranean well bores of the type commonly used for recovery of oil or gas. More particularly, this invention relates to an improved method and system for automatically reducing the well fluid circulation rate while circulating a kick out of a well in response to one or more of a plurality of monitored well or surface equipment conditions relating to the fluid circulation system.
Drilling subterranean wells typically includes circulating a drilling fluid (xe2x80x9cmudxe2x80x9d) through a fluid circulation system. The circulation system typically includes a drilling rig and mud treating equipment located substantially at or near the surface of the well. The drilling fluid may be pumped by a mud pump through the interior passage of a drill string, through a drill bit and back to the surface through the annulus between the well bore and the drill pipe.
A primary function of drilling mud is to maintain hydrostatic fluid pressure control of fluids in the formations penetrated by the well bore. Weighting agents may be added to a mud to achieve the desired mud density. Overbalanced drilling techniques typically practice maintaining a hydrostatic fluid pressure in the well bore and on the formation equal to or slightly overbalanced with respect to formation fluid pressure (xe2x80x9cpore pressurexe2x80x9d), both when circulating and when not circulating the mud. In underbalanced drilling techniques, hydrostatic pressure in the well bore is maintained at least slightly lower than formation pore pressure by the mud, supplemented with surface well control equipment. If the well bore encounters a zone having a substantially higher pore pressure than the hydrostatic fluid pressure in the mud, an influx of formation fluid may be introduced into the well bore. Such occurrence is known as taking a xe2x80x9ckick.xe2x80x9d
When a kick is taken, the invading formation liquid and/or gas may xe2x80x9ccutxe2x80x9d the density of the drilling fluid in the well bore annulus, such that as more formation fluid enters the well bore, hydrostatic control of the well bore may be lost. Such occurrence may be noted at the drilling rig in the form of a change in pressure in the well bore annulus, changes in mud density, and/or a gain in drilling fluid volume in the mud system tanks (xe2x80x9cpit volumexe2x80x9d). When a kick is detected or suspected, mud circulation is conventionally halted and the well bore closed in/shut in to measure the pressure buildup in the well bore annulus, pit gain and shut in drill pipe pressure. Appropriate well-killing calculations may also be performed while the well is closed in. Thereafter, a known well killing procedure may be followed to circulate the kick out of the well bore, circulate an appropriately weighed mud (xe2x80x9ckill mudxe2x80x9d) into the well bore, and ensure that well control has been safely regained. When a circulated kick enters long, narrow, and/or restrictive choke lines, such as may be encountered with a deepwater floating rig, the operator may anticipate this condition and briefly shut in the well.
Typically, the intent of the operator while circulating a kick out of a well is to hold pump rate constant at a normal or high rate, and only change the pump rate if an excessive or undesirable condition arises. It is common practice during the course of drilling the well bore to frequently measure and record the slower mud pump rates and corresponding pump circulation pressures required to circulate the mud. These slow mud pump rates, e.g., about one-half to one-third of the normal circulation rate while circulating a kick out of a well, may be used to more slowly and carefully circulate the kick out of the well bore. The cumulative number of pump strokes from a mud pump or a plurality of mud pumps required to circulate the hole may thus be measured or assumed, and is generally known to the well operator.
One of the most common techniques for killing the well and circulating an appropriate kill fluid is the xe2x80x9cconstant bottom hole pressurexe2x80x9d method, whereby bottom hole pressure may be maintained substantially at or above formation pore pressure. Two variations of this method exist: the Driller""s method and the Wait and Weight method. The Driller""s method may be utilized when kill weight fluid is not yet available for circulation. In the Driller""s method, the original mud weight may be used to circulate the contaminating fluids from the well bore. Thereafter, kill weight mud (xe2x80x9cKWMxe2x80x9d) may be circulated into the drill pipe and the well bore. Although two circulations may be required to effectuate the Driller""s method, this method may be quicker than the subsequently discussed variation.
In the Wait and Weight or xe2x80x9cEngineersxe2x80x9d method, KWM is prepared and then circulated down the drill string and into the well bore to remove the contaminating fluids from the well bore and to kill the well, in one circulation. This method may be preferable to maintain the lowest casing pressure during circulating the kick from the well bore and may thereby minimize the risk of damaging the casing or fracturing the formation and creating an underground blowout.
A substantially constant bottom hole pressure may be maintained in both methods. In both methods, pressure within the casing and/or drill pipe may be controlled by adjusting a choke conducting mud from the well to a mud reservoir. To further control pressure, the mud pump rate may be maintained at one of the previously measured or assumed low or reduced circulation rates. In the Driller""s method, a constant drill pipe pressure may be maintained during the first circulation, which may include the shut in drill pipe pressure (xe2x80x9cSIDPPxe2x80x9d) plus the slow rate pump pressure, plus a nominal safety factor, e.g., fifty psig. During the second circulation, the casing pressure may be held constant while the KWM is circulated to the bit, and then the drill pipe pressure held constant while the KWM is circulated from the bit to the surface. In the Wait and Weight method, a substantially constant bottom hole pressure may be maintained during the one circulation of KWM. KWM may be circulated down the drill string while maintaining drill pipe pressure at a calculated, predetermined pressure while the mud pump is maintained at a constant rate. The drill pipe pressure may gradually decrease as KWM is circulated to the bit. After KWM reaches the bit, the drill pipe pressure may be held constant until the KWM reaches the surface. A combination method is known which may combine portions of each of the above two methods. After the well is shut in and the pressures recorded, pumping of original weight mud may begin while the original weight mud is being weighted up to KWM, as the kick is being pumped out of the well bore.
Each of the above methods may be time consuming and may require extensive planning, calculations, monitoring, human intervention and/or coordinated regulation of components, rates and pressures during execution of the respective method. In addition, each method preferably uses a substantially constant pump rate (reduced circulation rate) in order to maintain control of the process during execution of the respective method, hopefully while maintaining a substantially constant bottom hole pressure. The Wait and Weight method also may require constructing a graphical or tabular pumping schedule of pump pressure versus volume pumped. In the event it becomes necessary to change pumping rates and/or interrupt pumping while executing the procedure to circulate the kick out of the well, it frequently may be necessary to record new shut in and circulating pressures, and recalculate a new pumping and/or pressure schedule. While circulating a kick out of the well, it is common for the drilling operator to monitor a plurality of signals relating to the fluid circulation system and, in response to one or more of those signals exceeding on an acceptable limit, the operator shuts the well back in and restarts the procedure of circulating the kick out of the well.
Following completion of the kill procedure, new pressure readings should be taken, wherein the well may be under hydrostatic control, such that the casing pressure may read substantially zero psig. A failure to maintain a constant bottom hole pressure may result from miscommunication, erroneous operation of the choke, procedural miscalculations, and/or other inappropriate equipment operation during the procedure. SPE paper 19245 by J. M. Prieur describes various well control issues in high temperature/high pressure wells. An article in SPE Drilling Engineer, December 1991, discloses sizing of a mud-gas separator to avoid problems.
The operator of a well fluid circulation system is in control of a facility where safety is paramount, and where an unnecessary shut down may be very costly. In many situations, the operator knows and understands the xe2x80x9cfeelxe2x80x9d of the well during the procedure to circulate out the kick of the well and, based on prior experience, is able to predict well operations with a similar set of symptoms. Shutting in a well for several hours while circulating a kick out of a well may cost thousands of dollars, and commonly leads to other problems, including excessive pressure and lost circulation to the formation, and reduced pressure which abruptly increases when a kick comes in. Rather than shut in a well during the procedure for circulating a kick out of a well after the well is shut in (or substantially shut in), particularly under circumstances where it is not necessary to shut in the well, it is preferable to keep the well circulating at a reduced rate.
The drilling operator is responsible for controlling the mud pumps, the chokes, and other surface equipment which affect fluid circulation. The drilling operator also serves a diagnostic function when complications or potential problems in a well are sensed. The drilling operator also is involved in managing well conditions, including various temperature, pressure, and flow rate conditions. The amount of human intervention required, including the substantial gathering of pump rate and pressure information, calculating and scheduling a kill procedure, maintaining a constant pump rate, and coordinating the operation of equipment to maintain the appropriate surface pressures and constant bottom hole pressure, are disadvantages of the prior art. An improved method and system for more accurately and reliably controlling well fluid circulation rate when circulating a kick out of a well are described below.
The present invention provides the operator with increased assurance that the circulation rate will be automatically reduced to a predetermined circulation rate in the event that a monitored condition exceeds an acceptable value. Prior performance of the well at that predetermined reduced circulation rate will provide further confidence to the drilling operator with respect to both the reduced circulation rate and the subsequent increase in circulation rate once the problem is eliminated.
A control system is provided to monitor one or more selected drilling parameters and provide automated control to reduce the fluid circulation rate to a selected value while circulating a kick out of a well based on the monitored parameters. The control system may monitor selected pressures, pump rates, choke position, pit volumes in the mud system, alarm conditions, lost circulation detectors, bit nozzle plugging detectors, choke washout detectors, mud-gas separation system operation, and/or sensor failure. When a potential problem is detected, i.e., a sensed condition rises above or falls below an acceptable value, the control system may be used to automatically reduce the fluid circulation rate while circulating the kick out of the well. In addition, the control system may facilitate returning the circulation rate to the normal rate in response to changes or interruptions in the pumping operation while circulating the kick out of the well.
It is an object of this invention to provide methods and systems for reducing the fluid circulation rate to a predetermined rate in response to monitored parameters while circulating a kick out of a well.
It is an object of the invention to provide an improved method of varying fluid pressure in a circulation system while circulating a kick out of the well. The circulation system conventionally includes a well bore drilled through a subterranean formation using a drilling rig, a drill string having a through bore and positioned at least partially within the well bore, a fluid pump for pumping a fluid through the drill string and into the well bore, and a drilling fluid choke in fluid communication with an annulus of the well bore. The method includes pumping of fluid through the drill string, then through the well bore annulus and substantially back to the drilling rig, with the fluid being pumped at a desired fluid circulation rate while circulating the kick out of the well. A plurality of sensors are used to monitor conditions of the circulation system while pumping the fluid. In response to the monitored conditions, the fluid circulation rate is automatically reduced to a predetermined reduced circulation rate.
It is a related object of the invention to provide an improved system for varying fluid pressure in a circulation system. The controller is responsive to a plurality of sensors for sensing conditions relating to the fluid circulation system, and automatically reduces the fluid pressure in the circulation system to a predetermined reduced circulation rate in response to the sensed conditions.
It is a feature of this invention to automatically measure and record drill pipe circulation pressures for a range of mud pump circulation rates, and use this invention to better control the process of circulating a kick out at a well.
A significant feature of the invention is that the flow rate from the fluid pump and the position of the drilling fluid choke are preferably automatically controlled when reducing the fluid circulation rate while pumping a kick out of the well. A related feature of the invention is that the controller may automatically compare monitored conditions to various fluid circulation rates from the fluid pump and the choke position at various circulation rates, and automatically reduce the fluid circulation rate to a predetermined reduced circulation rate in response to the monitored conditions. Still another feature of the invention is that the controller may automatically position the choke to hold the pressure constant, and the measured pressure may be compared with pressure data at that choke position and pump rate.
Still another feature of the invention is that the predetermined reduced circulation rate determined by the controller may be a function of a number of unacceptable monitored conditions. The lowest reduced circulation rate for any of the monitored conditions may be used as the controlling rate.
Still another feature of the invention is that the controller may increase fluid circulation rate to a desired high fluid circulation rate in response to monitored conditions after reducing the circulation rate to the predetermined reduced circulation rate. While at the reduced circulation rate, possible problems in the circulation system may be more easily detected.
Still another feature of the invention is that the monitored conditions may include two or more of an alarm condition, fluid circulation rate change, lost circulation, plugging of the bit nozzles, choke wash out, mud-gas separation system operation, fluid pressure in the circulating system, sensor failure, choke control operation, and fluid temperature in the circulation system.
It is an advantage of this invention to utilize an automated control system to better monitor and control the operation of the well circulation system while circulating a kick out of a well.
It is also a significant advantage of this invention to expedite the process of circulating a kick out of a well bore, thereby decreasing the time required to regain well control and decreasing well bore drilling costs.
It is further an advantage of this invention to improve control of surface equipment while circulating a kick out of a well by utilizing a control system to automatically regulate pump rates and choke positions.
It is an additional advantage of this invention to improve the safety of circulating a kick from a well bore utilizing a programmable control system. The control system may consider sensed measurements of well bore and drill string pressures, circulation rates, mud weight, and well bore dimensions, and in response to these monitored conditions, automatically reduce the fluid circulation rate, with reduced potential for miscalculation or manual control errors.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to figures in the accompanying drawing.